Machine-to-Machine (M2M) systems are now drawing much attention as a next-generation communication technology and the IEEE 802.11 WLAN is developing a new standard to support M2M systems. An M2M system refers to a network in which machines rather than people communicate information with other machines. Temperature sensors, humidity sensors, cameras, home appliances such as TVs, processing machines in factories, large machines such as automobiles may form a part of M2M systems. As various types of new communication services such as smart grid, eHealth, and ubiquitous communication emerge, M2M technology is frequently employed to support such services. Features of M2M systems may be summarized as follows.                1) Large number of stations: Different from the existing networks, M2M systems assume a large number of stations. It is because an M2M system should take account of all the sensors installed at home or office as well as machines owned by individuals. Therefore, a significantly large number of stations may be connected to a single AP.        2) Low traffic load for each station: Since an M2M device relies on a traffic pattern of gathering and reporting peripheral information, the information should not necessarily be transmitted often and the amount of information is also small.        3) Uplink-centric: In most cases, an M2M system receives commands through downlink transmission and takes relevant actions and reports resultant data through uplink transmission. Since most of important data are usually transmitted through uplink transmission, M2M systems can be considered an uplink-centric system.        4) Long lifetime of stations: A typical M2M device operates by battery and it may be difficult for a simple user to charge battery frequently. Therefore, M2M systems are required to minimize battery consumption, thus providing long lifetime of operation.        5) Automatic recovery function: Since in some special situations, it may be hard for a user to directly operate M2M devices, automatic recovery function should be implemented.        
In the M2M environment, an STA refers to a machine gathering and providing information; therefore, a pre-assigned AP/network should be able to control all of the M2M devices. However, operation protocol of present STAs may cause a problem that since STAs can perform switching between power saving and sleep/awake mode on their owns, from the viewpoint of a user operating a server (AP), M2M devices may stay in the sleep mode when they are expected to be operating or in the awake mode when they are not required, consuming battery power.